Dihydroxybenzene compounds have found wide application in industrial processes. Hydroquinone functions as a reducing agent in chemical reactions, and as an antioxidant in gasoline, rubber, plastics, and the like. Hydroquinone is useful in photographic chemistry, and hydroquinone derivatives are utilized as food antioxidants.
Ind. Eng. Chem. Prod. Res. Dev., 15, 212 (1976) by J. Varagnal describes several known processes for producing hydroquinone: (1) aniline is oxidized by magnesium dioxide to benzoquinone, which then is catalytically reduced to hydroquinone; (2) p-diisopropylbenzene is air-oxidized to a dihydroperoxide, which by acidic cleavage yields hydroquinone and acetone; (3) high pressure and temperature carbonylation of acetylene yields hydroquinone directly; and (4) benzene is electrochemically oxidized to benzoquinone which in turn is reduced electrochemically.
Of particular interest with respect to the present invention are processes in which aromatic compounds are hydroxylated with hydrogen peroxide in the presence of a catalyst.
U.S. Pat. No. 3,580,956 describes a process for nuclear hydroxylation of a substituted benzene or naphthalene compound which involves reacting the compound with hydrogen peroxide in contact with a faujasite or mordenite aluminosilicate catalyst. Phenol is converted into a mixture of dihydroxybenzenes and tar byproduct.
U.S. Pat. No. 3,914,324 describes a process in which phenol is hydroxylated with hydrogen peroxide in the presence of an alkanoic acid and an acid catalyst such as phosphoric acid or trichloracetic acid. The process yields a mixture of catechol and hydroquinone.
U.S. Pat. No. 4,301,307 describes a process for hydroxylation of aromatic compounds which involves reacting an aromatic compound with hydrogen peroxide in the presence of trifluoromethanesulfonic acid catalyst. Phenol is hydroxylated to a hydroquinone/catechol mixture with a 2:1 molar selectivity.
U.S. Pat. No. 4,396,783 describes a process for hydroxylating aromatic hydrocarbons which involves reacting an aromatic compound with hydrogen peroxide in the presence of a synthetic zeolitic catalyst such as titanium silicalite. Phenol is converted to an approximately equimolar mixture of hydroquinone and catechol, with an additional yield of tar byproduct.
There is continuing interest in the development of new and improved processes for selective oxidation of aromatic compounds.
Accordingly, it is an object of this invention to provide an improved process for selective hydroxylation of substituted aromatic compounds.
It is another object of this invention to provide a process for selective hydroxylation of phenol to hydroquinone.
It is a further object of this invention to provide a process for shape-selective hydroxylation of phenol to hydroquinone with hydrogen peroxide in the presence of a heterogeneous catalyst.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.